The subject matter disclosed herein relates to improvements to RAT startup.
A Ram Air Turbine (RAT) is controlled by a governing mechanism to maintain a narrow operating speed range. The governor spring and blade aerodynamic forces rotate the blades toward fine pitch orientations at which substantially rapid rotation occurs with blade faces oriented substantially perpendicularly with respect to the airstream. Blade counterweights rotate the blade toward coarse pitch to prevent over speed conditions. The various forces of the governor spring, the blade aerodynamics and the blade counterweights balance each other to maintain the proper speed range but the counterweights require centrifugal forces to develop their restraining forces.
During startup, centrifugal forces are low, so the RAT governor is typically controlled by the governor springs with the blades in the fine pitch position. The airfoil shape is optimized to give power over the operating speed range, so it is somewhat inefficient at low RPM and fine pitch and there is very little torque available to overcome the large blade inertia and the RAT tare losses at low RPMs. Indeed, even if the RAT starts to turn, it takes a long time for a fine pitch RAT to get up to operating speed. Moreover, RATs are sometimes placed in turbulent zones under the aircraft where the dynamic pressure loss is high. This further reduces the available torque for start up.
Both fine pitch and coarse pitch start up RATs are currently in production. The fine pitch architecture has proven superior for reliability, robustness, weight, cost and packaging. Coarse pitch RATs contain more heavy, complicated parts and more failure points, but they start up more efficiently.